In recent engine control systems, two catalytic converters are arranged in series in the exhaust system of an internal combustion engine. One catalytic converter is arranged on the upstream side in the exhaust system as an upstream catalyst and the other catalytic converter is arranged on the downstream side in the exhaust system as a downstream catalyst. In this system, an air-fuel ratio (A/F) sensor for linearly detecting an air-fuel ratio of fuel mixture is arranged upstream of the upstream catalyst. An oxygen sensor for detecting the air-fuel ratio on the upstream side of the downstream catalyst and an oxygen sensor for detecting the air-fuel ratio on the downstream side of the downstream catalyst are arranged upstream and downstream of the downstream catalyst, respectively. This system is disclosed in U.S. Pat. No. 5,537,817 (Japanese Patent No. 2869925) and Japanese Patent No. 2946379.
The above system performs a main feedback control for controlling an air-fuel ratio detected by the A/F sensor to a target air-fuel ratio. Further, a second feedback control for controlling an output voltage (second output voltage) detected by the oxygen sensor (second sensor) arranged downstream of the upstream catalyst to a second target voltage is performed. Still further, a third feedback control for controlling an output voltage (third output voltage) detected by the oxygen sensor (third sensor) arranged downstream of the downstream catalyst to a basic voltage is performed.
In the third feedback control, the third output voltage is so controlled to the basic voltage as to optimally keep the cleaning rate of the downstream catalyst. Then, in the third feedback control, in order to optimize the cleaning rate of the downstream catalyst, a second target voltage is set based on the output voltage of the third sensor. Further, in the second feedback control, in order to make the second output voltage follow the second target voltage set in the above manner, a target air-fuel ratio on the upstream side of the upstream catalyst is set. By keeping the cleaning rate of the downstream catalyst at an optimum value by the use of the second feedback control and the third feedback control, the air-fuel ratio of an exhaust gas discharged from the downstream catalyst is optimally controlled.
In order to optimally control the cleaning rate of the downstream catalyst, the second target voltage is set based only on the third output voltage. For this reason, even if the cleaning rate of the upstream catalyst is reduced, the exhaust gas supplied to the downstream catalyst is cleaned by the downstream catalyst. Therefore, the amount of harmful components in the exhaust gas finally discharged is reduced to a lower level.
However, the third sensor is arranged at the most downstream position of the engine in the exhaust system. For this reason, the third feedback control performed based on the output voltage of the third sensor has the largest delay in response among the three feedback controls. Thus, even if it is intended to reduce the cleaning rate of the upstream catalyst thereby to optimally keep the cleaning rate of the downstream catalyst, it is difficult to control the downstream catalyst at a proper cleaning rate with respect to an expected variation in the air-fuel ratio. Therefore, there is a possibility that the exhaust emission will deteriorate.
For example, in a case where the output voltage of the second sensor is stoichiometric and the output voltage of the third sensor is rich, the cleaning rate of the upstream catalyst is kept at an optimum value. That is, harmful components contained in the exhaust gas are significantly cleaned by the upstream catalyst. For this reason, optimizing the cleaning rate of the downstream catalyst is of less importance. In this condition, the second target voltage is changed by the third feedback control based on the third output voltage. As a result, the second output voltage is controlled at the target voltage by the second feedback control to reduce the cleaning rate of the upstream catalyst. As a result, it is likely that it will be difficult to keep the cleaning rate of the downstream catalyst at an optimum value.